Surfactants, which are amphipathic in essence, have useful applications in a broad spectrum of industries, including medicines, foods, cosmetics, etc, with classification into dispersants, emusifiers, solubilizers, foaming agents, anti-foaming agents, polishing agents, lubricants, surface-treating agents, wetting agents and so forth, depending on function and purpose. Because of their thermodynamic instability, surfactant products are manufactured by various methods associated with kinds and amounts of surfactants, solvents, viscosity, stirring, temperature, etc. Surfactants find numerous applications in various industries: one of the most active fields of research into surfactants may be found in the production of emulsion products. Nonionic surfactants in current use in the food, medicine and cosmetic industries may be representatively exemplified by POE sorbitan fatty acid esters, sorbitan fatty acid esters, glyceryl fatty acid esters, POE fatty alcohol ethers, POE alkylphenol, and POE-POP-POE copolymers, which are extensively used owing to their excellent surface activity.
Usually, in practice, the stability of emulsion products made of surfactants is obtained by using in HLB values of surfactants, volume ratio of inner phase to outer phase, and viscosity. However, it is difficult to keep emulsion products of very low viscosity stable for a long period of time. Particularly, it is hard to stabilize, for a long period of time at high temperature (37–40° C.), the products which are produced using such conventional surfactants at low concentrations by usual processes, i.e., with a homogenizer. In fact, there is a strong demand for emulsion products that are highly stable under various conditions, and active research has been directed to the development of surfactants which allow emulsion products of high stability to be produced by usual processes.
As a rule, surfactants are adsorbed to the interfaces or surfaces of aqueous solutions to significantly reduce the interfacial tension or surface tension of the solutions. The performance and functions of surfactants result not from individual surfactant molecules, but from the assembly of surfactant molecules. In water, surfactant aggregations unite themselves through physical interaction, forming a monolayer or other various structures, examples of which include micelles, vesicles, liposomes, bilayers, and multilayers. In aqueous solutions of sufficient concentrations, most surfactants ultimately form uniform single phases known as mesophases or liquid crystals. If a surfactant has a molecular structure more favorable for the formation of liquid crystals in a uniform single phase, the molecules of the surfactant can be more orderly arranged and thus, form a more stable structure at a less concentration.
It is very interesting that many biosystems show liquid crystal properties. Mesophase represented for liquid crystal is an intermediate state of liquid and solid, and is found in various sites of the body: sterols or lipid analogs may be in mesophase (Encyclopedia of Chemical Technology, 3rd, Ed. Vol. 14, pp 395–427, Wiley-Interscience, 1984, U.S.A.). Also, liquid crystals seem to play an important role in the structure and biochemical functions of cells. Acting as surfactants in the body, lipids have influence on the physiological functions of organs and tissues. It is no exaggeration to say that life cannot be maintained without surfactants. Cell membranes are composed mainly of amphiphiles among which phospholipids are prevalent. Amphiphiles organize themselves into bilayer structures in which the molecules are closely packed in such a way that hydrophilic moieties are directed outward while hydrophobic moieties are arranged inwardly, constituting the inner phase without contacting with the aqueous environment. Existing as the outermost layer of the human skin, the stratum corneum is made up of cells. The stratum corneum is physiologically active, but is considered to be a dead material because its cell nuclei are atrophied. However, the stratum corneum, which deserves to be an important example of liquid crystals, acts as a barrier, consisting of the lipids which are responsible for the transportation of water, chemicals, biologically active materials, etc. In the body, the liquid crystals are very important in physiological and medical aspects. It is difficult molecules to diffuse across liquid crystal membranes, but to do rapidly and anisotropically along liquid crystal layers as in a liquid phase. In consequence, liquid crystal phases are closely associated with physiological activity.
Studies into surfactant structures revealed that hydrophobic interactions are the major driving force for the self-assembly or aggregation of amphipathic surfactants in aqueous solutions and thus, the structural characteristics of surfactants are primarily determined by the hydrophobic alkyl region of the surfactant molecules. Based on this fact, the present inventors disclosed that vitamin E is well inserted into the ordered, dense lipid bilayers of cell membranes, acting to protect against the oxidation of the cell membranes, and is advantageous to the formation of liquid crystals owing to its structure consisting of a hard, flat chromane ring and a flexible phytyl group (Young-Dae, Kim and Byung-Jo, Ha, Cosmetics & Toiletries, 108, April, 63–78 (1993)).
In addition, with knowledge of the structural and functional properties of vitamin E, the present inventors also formulated the hypothesis that vitamin E could play an efficient role as a hydrophobic group if it is applied to surfactants, and finally invented POE vitamin E having surfactant activity as well as skin-soothing action and moisture retention and anti-oxidative effects to protect cells from harmful active oxygen by adding ethylene oxide (EO) to vitamin E (Korean Pat. No. 083024; U.S. Pat. No. 5,235,073; Japanese Pat. Appl'n No. Hei 4-10362).
By virtue of its structural characteristics, the POE vitamin E is well absorbed into the interface of aqueous and lipid phases, showing excellent surface activity. However, since the POE vitamin E is of low hydrophobicity owing to its relatively small hydrophobic alkyl tail and the presence of oxygen atom (O) at an internal position of the chromane ring, it can have constant HLB values with the use of a small hydrophobic moiety (Young-Dae, Kim and Byung-Jo, Ha, supra). This is a cause to make the cross sectional area of the surfactant small. Some of the POE vitamin E in which the hydrophilic moiety polyoxyethylene (POE) chain is short, i.e., the POE vitamin E of small sizes, is too well adsorbed into the skin owing to the instability of the short POE chain and the excellent surface activity. Thus, there is demanded an improvement in compatibility with the skin. In detail, where the POE chain alone is short, it is likely to degrade. Additionally, because the hydrophobic, flat, hard chromane ring moiety, which serves as a mesogen in the formation of liquid crystals, aligns one by one neatly while the phytyl tail has fluidity, the POE vitamin E is too readily inserted into the lipid bilayers of cell membranes, causing a problem in safe use on the skin. This safety problem may be overcome by controlling the length of the ethylene oxide chain of the surfactant, that is, by extending the ethylene oxide chain. In this case, however, the POE vitamin E is too hydrophilic to maintain predetermined HLB values, thereby a desirable surfactant function cannot be obtained.
In order to avoid the safety problem of the POE vitamin E which is superior in high surface activity, but low in the polymerization degree of EO, the present inventors performed various experiments. In one of the experiments, the structure of POE vitamin E was altered in such a way that the hydrophobic POP was joined to a terminal portion of the hydrophilic POE to produce POP-POE vitamin E in which the hydrophilic POE chain is interposed between two hydrophobic moieties, with the expectation that, the amphiphile would have an appropriate size ratio between hydrophobic and hydrophilic regions and thus could form vesicles like diacyl phospholipids which form liposomes with closed bilayer membranes. In addition to being superior in surface activity, the POP-POE vitamin E was found to have excellent ability to form liquid crystal membranes similar to biomembranes and be safe for use on the skin, as disclosed in Korean Pat. Appl'n No. 10-1998-20750. Despite these advantages, the POP-POE vitamin E does not afford the production of stable emulsion products with low viscosity which hardly undergo separation.